This invention relates generally to apparatus for receiving and combining together a plurality of modulated signals, and, more particularly, to apparatus of this kind that controllably weight the various signals being combined so as to null out an interference signal superimposed on each one.
Null processing receivers of this kind are useful in numerous applications. One example is a system for processing signals received by a multi-element antenna array in the presence of an interference (e.g., jamming) signal received from an unspecified, variable direction. In such a system, the modulated rf signals supplied by the various antenna elements are typically summed together to produce a sum signal for subsequent down-converting, demodulation and baseband processing. Prior to summation, each rf signal is controllably adjusted in amplitude and phase angle (i.e., complex weighted) so as to null or cancel out the presence of the interference signal in the sum signal. This adaptive interference cancelation is usually performed in a way that minimizes the sum signal's power, since it is assumed that the power of the interference signal greatly exceeds that of the desired information signal.
Since the direction from which the interference signal is received by the antenna elements can vary, the complex weighting must be controllably adjustable in order to maintain continuous nulling. This adjustment actually steers the spatial nulls present in the composite antenna pattern, to align a particular spatial null with the detected interference signal direction.
The modulated antenna signals whose amplitudes and phase angles are being continuously adjusted are at radio frequencies, typically L-band. Circuitry for effecting this adjustment typically includes highly sensitive microstrips, strip lines, and minute coils of wire, all of which can require sensitive trimming. Not only is such circuitry considered not entirely reliable, but it also is considered excessive in size, weight, power consumption and cost.
It should therefore be appreciated that there is a definite need for a null processing receiver of the kind described above that not only provides improved reliability, but also a reduction in size, weight, power consumption and cost. The present invention fulfills this need.